S. L. Penfield — Chemical Composition of Aurichalcite. 107 



formula of aurichalcite and as the mineral appeared to be of 

 unusual purity the following investigation was undertaken to 

 determine if possible its true chemical composition. 



As far as can be told from the small hand specimens in the 

 author's possession, the mineral occurs in narrow seams about 

 one centimeter wide in an impure limonite ; calcite was asso- 

 ciated with it, especially on one side of the seam and great 

 care was taken to pick out pure material. The aurichalcite 

 had the usual pale bluish-green color and occurred in radiated 

 tufts of microscopic crystals so soft and loosely aggregated 

 that a cluster of them could readily be pressed to a powder 

 between the fingers. No definite idea regarding its crystalli- 

 zation was obtained by examining under the microscope. It 

 was seen that it occurred in little flattened prismatic crystals 

 with mostly broken, irregular contours in general agreeing 

 with the description given by A. Belar.* 



After ascertaining that the mineral did not lose water by 

 heating at 100° C. the larger selected fragments were boiled in 

 water to expel the air from between the crystals and the 

 specific gravity taken very carefully on a chemical balance ; 

 the two portions were then analyzed separately with the fol- 

 lowing results. 



No. I. No. II. 



Weight of mineral 0-5690 0*3342 



Specific gravity 3'52 3'63 



C0 2 16-50 16-22 



CuO 20-88 19-87 



ZnO __ 52-18 5401 



H 2 9-91 CaC0 3 9*93 CaC0 3 



CaO -86 = 1-53 -36 = 0-64 



100-33 100-39 



The finely powdered mineral, weighed in a platinum boat, 

 was ignited in a combustion tube and the C0 2 and H 2 col- 

 lected and weighed in the ordinary absorption apparatus. The 

 copper was separated from the zinc by two precipitations from 

 strong hydrochloric acid solutions with hydrogen sulphide. 

 The larger percentage of CuO in the first analysis is not owing 

 to an incomplete separation of copper from the zinc, as proved 

 by dissolving the copper, after having weighed it, precipitating 

 it a third time and finding no trace of zinc in the filtrate. 

 The variation of CuO and ZnO in the two analyses indicates 

 the mutual replacement and isomorphism of the two oxides. 

 The CaO comes undoubtedly from an admixture of calcite. 

 Correcting the specific gravity and the percentages for 1*58 



* Zeitschr. Kryst., xvii, p. 113. 



